Optical resolution for lithography is determined by Rayleigh's equation. For the state of the art ArF lithography systems, the optical resolution is limited to 63 nm half pitch (HP) with a numerical aperture (NA) of 0.93 and K1 factor at 0.3.
Immersion lithography has also been proposed. Immersion lithography techniques replace the usual air gap between the final lens and a wafer surface with a liquid medium that has a refractive index greater than one. In such systems, the resolution may be reduced by a factor equal to the refractive index of the liquid. Current immersion lithography tools use highly purified water for the immersion liquid, and can achieve feature sizes below the Rayleigh limit. Immersion lithography, however, suffers from various manufacturing defects, such as, water marks, drying stains, water leaching, wafer edge peeling, and air bubbles that restrict full scale manufacturing efforts. Current development focuses on various manufacturing techniques that avoid these negative effects. The optical resolution for water-immersion lithography with an NA of 1.35 and K1 factor of 0.3 is limited to 42 nm HP, per Rayleigh's equation. Further research is being conducted to seek lens materials, immersion fluids and photoresists with higher index of refraction to further reduce the resolution limit. However, few breakthroughs have been reported making it an unlikely candidate as the technology of choice for the next generation lithography.
Currently, there are a number of lithography techniques under development that seek to provide optical resolution below the Rayleigh limit. For example, some have suggested employing a double patterning technique. Such a system may employ two exposures on two photoresist layers. There are technical challenges to employing a double patterning technique; for instance, alignment of the two patterns incident on the photoresist is difficult with current state-of-the-art scanners. Moreover, the process of depositing and etching with two photoresists as well as requiring two exposures increases the operation use of expensive scanners and thin-film processing tools.
Others have suggested using extreme ultraviolet (EUV) lithography as another solution to providing optical resolution below Rayleigh's limit for 193 nm optical lithography. Systems currently under development use 13.5 nm wavelength light sources. Various problems must be resolved before EUV lithography can be implemented; for example, low source power, contamination issues, and manufacturing and handling masks. These challenges have limited EUV lithography as a viable solution to optical resolutions below the Rayleigh limit.
Accordingly, there remains a general need in the art for a optical lithography system that can provide optical resolution below the Rayleigh limit.